style.css

body {
    display: grid;
    grid-template-columns: 1fr;
    grid-template-rows: 3rem 1fr 2rem;
    gap: 0px 0px;
    grid-auto-flow: row;
    grid-template-areas:
        "nav"
        "main"
        "footer";
    background-color: var(--background-color);
}

nav {
    grid-area: nav;
    display: flex;
}

nav a,
nav div {
    flex-grow: 1;
    height: 100%;
    display: grid;
    place-content: center;
    border: 0.5rem solid var(--background-color);
    border-top: 0px;
    transition: 0.3s;
    cursor: pointer;
    border-radius: 0 0 1rem 1rem;
    background-color: var(--background2-color);
    color: var(--text-color);
    text-decoration: none;
}

nav a:first-child,
nav div:first-child {
    border-left: 0;
    border-bottom-left-radius: 0;
}

nav a:last-child,
nav div:last-child {
    border-right: 0;
    border-bottom-right-radius: 0;
}

nav a:hover,
nav div:hover {
    background-color: var(--background3-color);
    color: var(--accent-color);
}

/* 
Main
*/

#main {
    grid-area: main;
    width: 100%;
    height: 100%;
    padding-bottom: 0.5rem;
}

/* 
Footer
*/

footer {
    grid-area: footer;
    background-color: var(--background2-color);
    display: grid;
    place-items: center;
}

footer a {
    color: #fff;
    text-decoration: none;
}

footer a:hover {
    text-decoration: underline;
}


/* Small screens */

@media screen and (max-width: 580px) {

    body {
        grid-template-rows: min-content 1fr 2rem;
    }

    nav {
        flex-direction: column;
    }

    nav a,
    nav div {
        border: 0;
        border-radius: 0;
        padding: 0.5rem;
    }
}